As use of mobile electronic instruments for consumer use, such as laptop personal computers and mobile phones, has become widespread, requirements of the miniaturization and the improved performance of such instruments are increased. In order to meet the requirements, recently, laminated substrates, in which multiple printed wiring boards are laminated, have been used, in addition to the densification of the printed wiring board.
The laminated substrate is a substrate obtained by electrically joining substrates in a manner in which filling a material for joining such as solder in via holes provided in each substrate and pressurizing the substrates while heating them. When the number of laminates of the substrate is three or more, if a joint, which has been formed from the material for joining when a first layer and a second layer are joined (on a first reflow), is melted and flows out of the joint when the resulting layers are joined to a third layer (on a second reflow), defects such as short circuit of connection are caused.
Even if the metal does not flow out, if the joint, which has been formed on the first reflow, is melted on the second reflow, defects may be caused, for example, the joint is damaged and the substrate is out of position. For that reason, materials for joining, which can form a joint having a certain level of joint strength as well as do not flow out on the second reflow are required. Then, materials for joining capable of providing a joint formed on the first reflow, which does not re-melted on the second reflow by heating the material for joining to a high temperature (making the material to have a high melting point) during the first reflow process, are used.
For example, when both of a heating temperature in the first reflow and a heating temperature in the second reflow are the same 250° C. to 270° C., the material for joining, which joints the first layer to the second layer, may be melted, but it is required for the joint, which has been formed in the first reflow, not to be melted at a temperature of 270° C., which is the highest temperature in the second reflow.
As such a material for joining having a high re-melting temperature, Patent Literature 1 discloses a solder paste obtained by mixing Cu balls with Sn balls in a flux. At a temperature equal to or higher than a melting point of Sn, the solder paste forms a compound including Cu6Sn5 from a part of the Cu balls and the Sn balls, and the Cu balls are bonded to each other through the compound including Cu6Sn5, and thus the re-melting temperature thereof is increased. According to this invention, when the Sn balls are fused, Sn is wetly spread on the Cu balls, fills spaces between the Cu balls, and exists comparatively uniformly between the Cu balls, whereby the Cu6Sn5 having a melting point of 400° C. or higher is formed on at least a part of the surfaces of the Cu balls, and the Cu balls are bonded to each other through the Cu6Sn5.
Patent Literatures 2 and 3 disclose that an intermetallic compound powder is previously added to a solder paste, and, at the same time, it is essential to add a copper fine powder